This project seeks to specify how ultradian, circadian and seasonal rhythms of androgen availability influence the development and subsequent maintenance of mammalian reproductive structures and behavior. The model species is the Siberian hamster, Phodopus sungorus sungorus. The number and duration of daily testosterone pulses necessary to induce and maintain sexual behavior and structure in male mammals is unknown. Nor do we understand how environmental factors present at different seasons impact on development of reproductive structures. To correct these deficiencies we aim to 1) determine the minimum daily durations of androgen treatments that support mating behavior in males and the manner in which hormone pulses synergize to control penile development, 2) establish the different androgen requirements for initial induction versus subsequent maintenance of sex behavior, 3) assess the relative contributions of local versus systemic actions of androgens on penile growth, 4) specify the neural circuit that mediates pulsatile effects of steroids on sex behavior and 5) determine whether effects of testosterone are mediated by androgen and/or estrogen receptors. In establishing the minimal number, lowest dose, and optimal spacing of daily androgen treatments necessary for normal reproductive development, the present work should lead to new insights and guidelines for treatment of children with insufficient phallic development, as well as treatment of adult hypogonadal men. This line of research should promote development of effective physiological interventions that will minimize undesirable side-effects associated with current pharmacological treatments for human disorders of androgen secretion.